by Lynn YarrisThe world record for field strength in a
dipole magnet has been shattered by researchers at Berkeley Lab. A
one-meter long superconducting electromagnet, featuring coils wound out
of 14 miles of niobium-tin wire, reached a field strength as high as
13.5 Tesla, far surpassing the previous high of 11.03 Tesla set by a
Dutch group in 1995.

The team that designed, built and tested the new record-holding
magnet was led by Ron Scanlan, a materials scientist with Berkeley
Lab's Accelerator and Fusion Research Division. The field strength
achieved by Scanlan and his group is about a quarter of a million times
stronger than the magnetic field of Earth and about triple the strength
of the superconducting dipole magnets at the Tevatron, the highest
energy particle accelerator in the world.

The new magnet is among the first to use a niobium-tin alloy for the
superconductivity (the absence of electrical resistance) of its coils.
Construction costs for this prototype were so high - about one million
dollars - that only one magnet could be built and tested. This was a
nerve-wrenching departure from the conventional practice of building
several test magnets at once.

"We were in unknown territory and even though we carefully tested
all of the components during construction, we could not know for
certain what we had until we tested the completed magnet," Scanlan
says. What he and his group have will likely serve as the model for the
dipole magnets that will be used in the next generation of high-energy
particle accelerators. This particular new dipole magnet will be used
at Berkeley Lab as a test facility for evaluating superconductors that
could yield even more powerful magnets in the future.

Dipole magnets are used to bend and maintain the path of
accelerating particle beams. The higher the field strengths of the
magnets, the tighter the arc of the beam. With stronger dipole magnets,
an accelerator can push particles to much higher relativistic energies
around the same-sized circular beam path. The use of high-field
strength superconducting electromagnets has always been a considerable
technical challenge, however, because superconductivity has a tendency
to weaken and disappear in the presence of a strong magnetic field.
Nonetheless, the inherent limitations of conventional electromagnets -
they cannot attain a dipole field strength much above 2 Tesla - has
prompted a continuing development of new and better superconducting
alloys.

In recent years, the alloy of choice for accelerator magnets has
been niobium-titanium. Superconducting magnets made from this alloy
operate in all of today's most powerful machines and will be used in
the Large Hadron Collider (LHC) now being built at CERN. The LHC
magnets are expected to operate at a field strength of 8.6 Tesla which
is approaching the 10 Tesla mark that is considered to be the upper
limit of niobium-titanium accelerator magnets.

In the search for superconductors capable of reaching higher field
strengths, it was determined that niobium-tin could, in principle, fit
the bill. However, unlike niobium-titanium, niobium-tin is a
non-ductile material, and was thought to be too fragile and brittle to
withstand the stress of fabrication.

Scanlan and his group overcame the brittleness obstacle by making
their cable from separate strands of niobium and tin in a copper
composite strand (fabricated by the companies of Intermagnetics General
and Teledyne Wah Chang) while the materials were still ductile. Only
after they wound their cable into four magnet coils did they meld the
separate niobium and tin strands into the superconducting compound that
is so brittle. The alloy was made by heating the coils to 950 Kelvins
(about 680 degrees Celsius), baking them for about ten days, then
cooling the material to ambient temperature. Once the four coils were
assembled into a dipole magnet they had to be cooled far below room
temperature to make them superconducting, this time to a temperature of
about 4.3 Kelvin (-270 degrees Celsius). "The thermal
expansion-contraction effects in going from a reaction temperature of
950 K to a test temperature of 4.3 K are enormous," says Scanlan. To
withstand this and other stresses, the wound coils are impregnated

After being filled with epoxy, each coil is encased in an iron yoke
that contributes to the strength and stability of the magnetic field.
The coils are then wrapped in 18 layers of sheet stainless steel,
forming a collar that prevents the coils from separating under the
force generated when their tremendous magnetic field is energized. The
finished meter-long magnet is also about one meter in diameter and has
a 50 millimeter bore. It weighs about seven tons.

"Inside the iron yoke and stainless steel wrap of each coil is
something that is as brittle as glass," says Scanlan, explaining why,
despite its bulk and solid appearance, the magnet has to be handled
with great care when it is moved from one location to another.

As with any new superconducting electromagnet, the niobium-tin
dipole at Berkeley Lab had to be "trained" to attain its peak field
strength. Training is a staggered process in which the magnet is
chilled until its coils become superconducting (using liquid helium)
then energized up in field strength until superconductivity is lost in
some parts of its coils through inadvertent warming. This temporary
loss of superconductivity is called "quenching" and when it occurs, the
magnet must be given time to recover, then re-cooled.

"The magnet was ramped to slightly above 10 Tesla before the first
quenching occurred," says Scanlan, calling this initial effort
"encouraging." The magnet was then slowly but steadily trained upward
in field strength until, after 13 quenches, it reached 11.14 Tesla.
When further training at 4.3 Kelvin failed to raise the field strength,
Scanlan and his group began lowering the temperature. Field strength
maxed out at the 13.5 Tesla mark after the temperature had been dropped
to about 1.8 Kelvin (-271 degrees Celsius). The entire process lasted
about three weeks.

"This became a test of the magnet's structural integrity as well as
its field strength because of the stress involved," Scanlan says. "Any
weakness in the structure would have caused the entire magnet to
fail."

The definition of a good magnet is one that stays trained, according
to Scanlan. Consequently, his team is now in the process of allowing
the magnet to warm to room temperature before cooling it back down and
ramping it up again. Based on performances during the earlier training
session, the magnet is expected to do fine. Eventually, Scanlan says,
his group will disassemble their magnet and rebuild it with an eye
toward bringing expenses down so future production costs will be
comparable to those of today's niobium-titanium magnets.